1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a liquid crystal display device having a wide viewing angle which is suitable for use in a flat display in a personal computer, a word processor, an amusement apparatus or a TV, or in a display apparatus utilizing a shutter effect.
2. Description of the Related Art
Known techniques for improving or widening the viewing angle of a liquid crystal display device include: moving the liquid crystal molecules only in a direction substantially parallel to the substrate surface; and dividing one pixel into a plurality of regions respectively having different orientations of liquid crystal molecules while the liquid crystal molecules move in a direction vertical to the substrate surface. A representative display mode which uses the former technique is the IPS (In-Plane Switching) mode. Representative display modes which uses the latter technique include: a wide viewing angle liquid crystal display mode (Japanese Laid-Open Publication No. 7-120728) where an Np (Nematic positive) type liquid crystal material is in an axially symmetric horizontal orientation; another wide viewing angle liquid crystal display mode (Japanese Laid-Open Publication No. 7-28068) where a vertically-oriented Nn (Nematic negative) type liquid crystal material is divided into regions and oriented differently in different regions by controlling an electric field applied therethrough; and still another wide viewing angle liquid crystal display mode (disclosed in AM-LCD'96, p. 185 (1996)) where an Np (Nematic positive) type liquid crystal material in each pixel is divided substantially equally into four regions, with the liquid crystal material being in different parallel orientations in each region.
Generally, in the latter technique, where each pixel is divided into a plurality of regions having liquid crystal molecules of different orientations, the viewing angle characteristic along an axial direction which extends equiangularly between a polarization axis 202 of an upper polarizing plate and an absorption axis 203 of a lower polarizing plate (which interpose a liquid crystal cell 201 therebetween as illustrated in FIG. 23B) is considerably inferior to the viewing angle characteristic along one of the absorption axes. Referring to FIG. 23A, a polar coordinate system is defined where: θ represents an angle from a direction normal to an imaginary plane 204 parallel to the liquid crystal cell to a viewing direction 205 from which the viewer is viewing the display; and Φ denotes an azimuth of the viewing direction 205 with respect to the absorption axis 203 (where Φ=0°) of a lower polarizing plate. When the viewing angle characteristic is evaluated in such a polar coordinate system, an isocontrast contour curve, as illustrated in FIG. 24, results generally irrespective of the display mode. Thus, the viewing angle narrows as the azimuth Φ shifts from the absorption axis of the upper or lower polarizing plate. A curve 302, also illustrated in FIG. 24, represents an isocontrast contour curve which the present invention aims to obtain.
The inventors of the present invention disclosed (Japanese Laid-Open Publication No. 7-120728) an ASM mode (Axially Symmetric Aligned Microcell Mode) where the liquid crystal molecules are oriented in an axially symmetric pattern in each pixel. In this mode, the phase separation of a mixture of a liquid crystal material and a photocurable resin is used to orient the liquid crystal molecules in an axially symmetric pattern. The Np type liquid crystal material used in this mode is such that the axially symmetric liquid crystal molecules are oriented vertically to the substrate by applying a voltage thereto.
A liquid crystal display device of this conventional ASM mode uses a liquid crystal material whose dielectric anisotropy Δε has a positive value. This display mode exhibits excellent display characteristics in any direction since the liquid crystal molecules are oriented in axial symmetry. However, when the respective absorption axes of the polarizing plates are arranged in a crossed-Nicols arrangement, the viewing characteristic tends to deteriorate. Moreover, a black matrix having a large area is required for preventing light from leaking in an OFF state. Furthermore, this conventional ASM mode utilizes a phase separation process which requires a complicated temperature control in order to realize the axially symmetric orientation. Therefore, a liquid crystal display device of this ASM mode is difficult to produce and the obtained axially symmetric orientation is unstable which is not reliable, particularly at a high temperature.
In order to solve such problems, the inventors of the present invention proposed (Japanese Laid-Open Publication No. 8-341590) a liquid crystal display device where the liquid crystal molecules are oriented in axial symmetry in each pixel region, which thus has a high contrast and exhibits excellent display characteristics in any azimuth, and a production method which allows such a liquid crystal display device to be produced easily.
The liquid crystal display device has: a pair of substrates interposing a liquid crystal layer therebetween including liquid crystal molecules with a negative dielectric anisotropy (Δ∈<0); and a vertical alignment layer on one surface of each substrate adjacent to the liquid crystal layer. Protrusions are further provided on at least one of the substrates so as to surround each pixel region. Moreover, a pair of polarizing plates are provided between the pair of substrates so that the respective absorption axes of the polarizing plates are perpendicular to each other. This liquid crystal display device does not require a complicated production process, but still realizes such an orientation where the liquid crystal molecules are oriented substantially perpendicular to the pair of substrates in the absence of an applied voltage, and are axially symmetric in each pixel region in the presence of an applied voltage.
In this liquid crystal display device, in the absence of an applied voltage, the liquid crystal molecules are oriented substantially perpendicular to the pair of substrates, thereby realizing a satisfactory black state and thus a high contrast display in a viewing angle normal to the display plane. From different viewing angles, however, light leakage is observed, and the contrast ratio is deteriorated, because (i) some of the viewing angle dependency results from an inherent characteristic of the polarizing plate, and (ii) the retardation value of the vertically-oriented liquid crystal molecules varies from one direction to another, thereby causing the retardation value of the liquid crystal layer to have a viewing angle dependency.
Hereinafter, the viewing angle dependency resulting from an inherent characteristic of the polarizing plate will be explained. When light is incident upon the above-described wide viewing angle mode liquid crystal display device from a direction of the polarization axis (transmittance axis) of the polarizing plate and passes through the refractive index ellipsoid of the liquid crystal layer, then, such light only contains a normal light component or an abnormal light component. However, when light is incident upon the device from a direction shifted by 45° from the absorption axis of the polarizing plate and passes though the refractive index ellipsoid, then, such light contains both normal and abnormal light components and thus is elliptically-polarized light. In such a case, apparent light leakage increases as the direction of the vibration of the polarized light shifts from one of the absorption axes of the polarizing plates, which are perpendicular to each other.
Hereinafter, the viewing angle dependency resulting from the varying retardation value of the liquid crystal layer will be explained. In the above-described liquid crystal display device, the liquid crystal molecules are oriented substantially vertically to the pair of substrates in the absence of an applied voltage. Accordingly, the retardation value varies depending upon the direction from which the display is viewed, whereby the viewing angle dependency is observed.
The viewing angle characteristic is particularly poor in a direction at about 45° from both of the absorption axes of the polarizing plates which are perpendicular to each other. The poor viewing angle characteristic occurs because in such a direction the inherent characteristic of the polarizing plate and the varying retardation value both affect the viewing angle characteristic of the display device. For example, in a direction at about 45° with respect to the absorption axis of the polarizing plate, the contrast of the display device considerably deteriorates over a certain viewing angle range, e.g., from about 35° to about 50°, where the gray-scale level is inverted. Thus, the display characteristic greatly deteriorates particularly in a grayscale display.